The present invention refers to a method for forming an air laid fibrous web, wherein an air born fiber stream is laid on a web-shaped forming member, such as a wire, and the air laid fibrous web is bonded together.
When air laying a fibrous web there are normally used one or more fiber distributors by means of which the air born fiber stream is distributed over a flat wire, which is under a vacuum, at which a fibrous web is formed. The fibrous web is compacted and bonded by means of moisture or a bonding agent, such as latex or bonding fibers. The bonding agent can either be contained as a component in the fiber stream or be added to the fibrous web after the air laying thereof.
Such dry or air formed materials obtains a high bulk, softness, smoothness and drapability as compared to a corresponding wet laid material and obtains almost textile like properties. Due to the bonding agent a high wet strength is also obtained. The drawback of the air laying process is that it is not possible to run in the high production speeds that are possible in a wet laying process. This involves a considerable higher production cost, which leads to that the method usually is limited to the production of relatively exclusive materials such as dinner napkins, table cloths, washing cloths and the like. The materials are also often used as pail of other hygiene products, like sanitary napkins and panty liners.
The production speed of such air laid fibrous webs is limited, mainly due to the fact that the unbonded web is very sensitive to all kind of mechanical influence which may lead to web breaks at free draughts and cause an uncontrolled rearrangement of the fibers and thus influence the fiber distribution in the web. Besides problems occur with dusting from the open fibrous web.
An object of the present invention is to provide a method for forming an air laid fibrous web of the kind mentioned above, and which permits production at considerably higher speeds than in conventional air laying technique on a flat wire and where the above mentioned problems are avoided. This has according to the invention been provided by feeding the air laid fibrous web between two web-shaped forming members and binding the fibrous web either when this is located between the two web-shaped forming members or immediately after one forming member has left the fibrous web.
According to one embodiment the fibrous web is exerted to a mechanical actuation when located between the forming members, in order to provide a more even fiber distribution. By this a very even fiber formation can be obtained by simple means.
According to a further embodiment at least one of the web-shaped forming members has a three-dimensional structure which is shaped into the fibrous web. Preferably a pressure is applied to the forming members in order to press the structure into the fibrous web.
According to one embodiment the air born fibrous stream is applied in a nip between two wires which are brought together immediately after the laying of the fibers. Preferably the wires are brought together over a curved forming element.
According to a further embodiment the air born stream is laid on a first substantially flat forming member, after which a second forming member is brought together with the first forming member with the air laid fibrous web therebetween.
According to a further embodiment the air born fibrous stream is fed between two forming members which are brought together gradually for admitting deaeration of the fibrous web over a relatively large area, at which the first part of the forming takes place over an open zone between forming members and the final part takes place over a closed zone where the forming members have been brought together. The mechanical agitation of the fibrous web takes place either as well in the open as in the closed zone or only in one of them.
The mechanical agitation of the fibrous web is done by a direct actuation thereof and/or by an indirect actuation via at least one of the web-shaped forming members.
Said agitation can be provided by means of vibrations, breaking over one or more rolls, air pulses, alternating vacuum and over pressure and/or by ultrasonic, infra sonic etc.
According to a preferred embodiment bonding of the fibrous web takes place when this is located between the wires.